This invention relates to inverted telephoto type zoom objectives, particularly those in which the air space between the divergent front and the convergent rear lens groups varies throughout a range of magnifications.
Various types of zoom lenses have been developed for the purpose of preserving image quality when the lens is zoomed to wide angle positions. These lenses may be classified into two main groups, depending upon the difference in aberration corrections, namely, one which is called the "positive lead form" in which the frontmost component is in the form of a positive lens and another group which is called the "negative lead form" in which the frontmost component is in the form of a negative lens. Examples of the positive lead form appear in U.S. Pat. Nos. 4,169,660, 4,142,779, and 4,147,410. On the other hand, examples of the negative lead form are shown in U.S. Pat. Nos. 3,848,969 and 4,155,629.
The present inventor has proposed a zoom lens of the negative lead form in Japanese Open Patent Sho 53-132360 (U.S. Patent application, Ser. No. 898,807). In the embodiments described in this patent application, the front lens group must be reciprocated to zoom the lens throughout its entire range. This imposes a significant limitation on the construction of the lens barrel mechanism. If the lens could be zoomed over its full range with a linear axial movement, and the front and rear lens groups could be varied unidirectionally in relation to each other, it would be possible to employ a control mechanism with an actuator which zooms the lens by imparting a straight movement thereto.
The chief point to be considered in this connection is that the prior art has already developed zoom objectives for adaptation to such control mechanisms, and they have the advantage that the optical system, when zoomed to the telephoto position, assumes its shortest physical length. Such zoom objectives have the drawback that the diameter of the frontmost lens is increased substantially when the image angle is increased in the wide angle position, or when the focusing range is extended to include shorter distances.